As analog circuits have been integrated with digital circuits on complementary-Metal-Oxide-Semiconductor (CMOS) integrated circuits, the capacitor has come to dominate analog circuit design. In many cases, capacitor devices consume a large part of an integrated circuit's total area. As a result, decreasing the size of capacitor devices will allow an integrated circuit to be smaller, and, thereby, allow the integrated circuit to be produced more cost effectively.
Integrated circuits typically contain one or more of three types of capacitors. The first type is a Metal-Oxide-Semiconductor (MOS) capacitor. In such a device, the near-surface region of a doped semiconductor substrate acts as one terminal of the capacitor. The gate conductor is used as the other terminal, and the gate oxide acts as the capacitor dielectric.
The second type of capacitor is formed using two or more metal interconnects. Typically, two metal lines are electrically biased to opposite polarities and are placed in close proximity to one another in order to form the terminals of the capacitor. A dielectric material such as silicon dioxide fills the region between the interconnects. The metal lines may be interdigitated to increase the effective capacitive area of the device. An example of an interdigitated metal interconnect capacitor can be found in U.S. Pat. No. 6,383,858 to Gupta et al., which is incorporated by reference.
Finally, the third type of capacitor comprises a Metal-Isolation-Metal (MIM) capacitor. In such a device, a regular metal interconnect feature acts as one terminal of the capacitor. A specially deposited thin dielectric and a specially deposited metal level act to create the dielectric and second terminal, respectively.
Each of the three types of capacitors has substantial limitations when used alone. Among the above-mentioned devices, the MOS capacitor provides the highest capacitance density, typically 4-13 fF/μm2 in 0.13 μm and 90 nm technologies, depending on the gate dielectric thickness. However, MOS capacitors do not make use of the space above the capacitor for creating additional capacitance. Typically, the space above the MOS capacitor is blocked off from levels of metallization.
Moreover, capacitors created by metal interconnects suffer from low capacitance and, as a result, by themselves are not an area-efficient way to create capacitive devices on integrated circuits. Typical capacitance density is approximately 1-2 fF/μm2. Likewise, MIM capacitors have low capacitance density, approximately 1 fF/μm2, and, as a result, are also not area efficient. Moreover, the forming of MIM capacitors requires at least two additional lithographic masks and their associated processing. Cost of implementation, therefore, may be very high.
For the foregoing reasons, a new capacitive device with a higher capacitance density and without additional implementation costs is highly desirable.